This invention relates to a method for controlling the characteristics of a sheet which is calendered from a temperature-critical thermoplastic material.
The method which generally is used to form thin sheets from thermoplastic material is to heat the material until it reaches a plastic state, force it through an extrust on die to form a homogeneous sheet, and then calender it to achieve the desired thickness and to impart any required contouring or special surface finish to it. Not only is calendering necessary where the thickness of the sheet is critical or where a special surface finish is required, if the resulting sheet is to be very thin, this sequence is much faster than extruding directly to the final thickness. Moreover, extruding directly to a very thin gauge generally will not provide a uniform product. Thus, calendering normally is the final step in producing thin sheets of thermoplastic material.
Since thermoplastic material must remain in a plastic state when it is being calendered, provisions must be made to maintain the calender rolls at the desired temperature. While this does not present any difficulty with many materials, some materials, such as ultrahigh molecular weight polyethylene compositions, can only be calendered if they are maintained within extremely narrow limits of temperature. If the surface temperature of the calender rolls should exceed these limits the material will stick to them, and if the surface temperature drops below these limits the resulting sheet will be brittle. Due to the narrow temperature range where such material can be calendered it is necessary that the rolls be heated at times and cooled at other times, and it may even be necessary to simultaneously cool portions of the rolls while other portions are being heated.
In the past, attempts have been made to maintain the temperature of the rolls within these close limits by convective heat transfer from heated and/or cooled oil or water which flows through them. If this fluid is allowed to flow unrestricted through the rolls at a low velocity there will be poor heat transfer and thus tremendous flow rates are necessary to transfer sufficient energy to give the quick response necessary to maintain the temperature within the required limits. Therefore, flow restrictors have been utilized to increase the rate of heat transfer and to provide surface temperature uniformity, making the rolls difficult and expensive to fabricate.
In addition, convective heat transfer requires a temperature differential between the fluid and the wall it is transferring energy to, and the greater the rate of heat transfer required the greater this temperature differential must be. Therefore, if the wall is being heated the fluid probably will initially be brought to a temperature which is greater than the temperature that the wall is to be heated to and then gradually cooled as the wall temperature approaches the desired temperature. Not only does this result in slow reaction to changes, if the wall temperature is not uniform and heating is generally required, there will be occasions where heat will be transferred to local hot spots which already are over temperature. These hot spots would not be cooled then until the system reached a point where very little overall heating or even cooling was required. Furthermore, with convective heat transfer the fluid either looses or gains in temperature as it flows through the rolls depending on whether it is heating or cooling. Thus, by its very nature convective heat transfer of the type used in the prior art devices results in a temperature gradient being created along the rolls. Finally, due to the difficulty in maintaining oil or water at a constant temperature, these devices require complex feed-back systems.
For the foregoing reasons, the prior art processes are slow in reacting to changing demands between heating and cooling, do not provide uniform surface temperature, and cannot cool local areas of the rolls while the rolls are in general being heated.
What is needed therefore is a calender roll which is simple and inexpensive to manufacture and yet which allows control of its surface temperature within a narrow range uniformly over its entire extent.